The field of the invention pertains to very hard, very tough ceramic materials for industrial tooling, and, in particular, to ceramic materials that can be formed into high temperature cutting tools, extrusion and drawing dies and wear parts.
Beginning about sixty years ago a variety of "engineered" ceramic materials having important industrial advantages, particularly in tooling, have been developed. Typically, particles of ceramic have been bonded together at low temperature with a resinous material or at a high temperature with a metal or other ceramic. Carbide grinding wheels are an example. Or the ceramic constituents have been combined at low temperature and hot pressed or sintered in a variety of processes for cutting tools and wear parts. Numerous patents have issued with the following of particular relevance to the invention disclosed below.
U.S. Pat. No. 4,104,075 discloses TiN based refractories with or without TiC, SiC and Al.sub.2 O.sub.3 as ingredients, and methods for using ores to react and form the ceramics into a bonded composite. Coarse refractory grains are retained in the matrix. U.S. Pat. No. 4,158,687 discloses a method for making ceramic composites containing continuous fibers of SiC. The continuous fibers are in preference to SiC whiskers which at the time of the patent were not of uniform quality and therefore produced inferior whisker reinforced ceramic products. In the intervening years since this patent, SiC whiskers have become available in uniformly good quality. As a result composite ceramics reinforced with Sic whiskers have been developed such as disclosed in U.S. Pat. No. Re. 32,843 wherein the whiskers are combined with Al.sub.2 O.sub.3, mullite or B.sub.4 C.
As an alternative U.S. Pat. No. 4,612,296 discloses Si.sub.3 N.sub.4 ceramics with silicides and carbides of a variety of metals added to the matrix in small plate form before sintering. The small plates are added to improve the toughness of the sintered body. TiN and TiC are included as toughening agents in small plate form. However, the use of SiC whiskers with surfaces treated to remove oxides and to add a carbon coating are disclosed in U.S. Pat. No. 4,916,092. The treatment purposely causes a lack of chemical bonding between the whiskers and the matrix thereby causing propagating cracks to deflect. The result is a tougher ceramic having only intermediate strength.
U.S. Pat. No. 4,867,761 discloses Al.sub.2 O.sub.3 cutting inserts reinforced with SiC whiskers limited to less than 15% by volume and the balance of the whiskers of titanium or zirconium nitrides or borides. However, the examples disclosed used TiN whiskers exclusively. The TiN whiskers are added to increase the toughness of the cutting inserts in cutting steel. U.S. Pat. No. 4,852,999 discloses TiC whiskers in an Al.sub.2 O.sub.3 matrix for cutting inserts with increased fracture toughness in cutting steel.
As a part of the development of ceramic engineered materials in recent years, electrically conducting ceramics have evolved. U.S. Pat. No. 3,808,012 discloses an electrically conducting ceramic of TiB.sub.2, B.sub.4 C and SiC and several methods of forming the ceramic into useful shapes. More recently patents have issued disclosing ceramic electrically conductive heaters. U.S. Pat. No. 4,528,121 discloses such a ceramic of predominantly Al.sub.2 O.sub.3 with TiN, TiC, TiB.sub.2 or other metallic compounds to impart electrical conductivity. U.S. Pat. No. 4,555,358 discloses SiC ceramic heater with zirconium or titanium borides and nitrides added to impart electrical conductivity. And U.S. Pat. No. 4,613,455 discloses Si.sub.3 N.sub.4 ceramic heaters with TiN and TiC added to impart electrical conductivity.
U.S. Pat. No. 4,507,224 discloses two varieties of electro-conductive ceramics specifically for electro-discharge machining. The first variety of electro-conductive ceramic comprises the addition of 5 to 50% by weight of dispersed SiC whiskers in a matrix of oxide ceramics, the whiskers ranging 10 to 50 microns in length. Surprisingly, this combination is disclosed to be exceptionally electro-conductive despite the very high resistivity of both the ceramic oxides and SiC whiskers. At the time this technology was developed, available SiC whiskers were highly contaminated with metal residues from the manufacturing process which may explain the electro-conductivity found and disclosed.
The second variety disclosed in U.S. Pat. No. 4,507,224 includes the further addition of electro-conductive carbides, nitrides and borides in a range of 2 to 20% by weight. The electro-conductive carbides, nitrides and borides are limited to 20% by weight because of the deleterious effect on strength. In both varieties the preferred range of SiC whisker length is 50 to 500 microns with lengths less than 10 microns considered deleterious because the large amount of SiC whiskers added to achieve electro-conductivity impairs the inherent properties of the ceramic if the short lengths of whiskers are used. Likewise the patent teaches whisker diameters of 0.1 to 10 microns with 0.5 to 3 microns preferred.
Since 1985, the manufacturers of raw SiC whiskers have significantly improved overall quality in terms of amount of non-whisker material, contaminants, chemistry and trace elements or compounds. Most dramatic has been the reduction in metal contaminants such as Fe, Cr, Ni, Mg and notably Co. Current SiC whiskers have very high inherent electrical resistance due to the low level of metal content, on the order of 0.02-0.03 wt. percent. An alumina composite containing 50 volume percent SiC whiskers of this cleanliness has exhibited a bulk resistance of 10.sup.6 ohm-cm versus the 10 ohm-cm disclosed in U.S. Pat. No. 4,507,224. An electrical resistance range of 10.sup.2 -10.sup.4 ohm-cm is typical for currently produced 40 volume percent SiC whiskers in an alumina matrix powder.
Accordingly, it is necessary to add an electro-conductive compound to the non-electroconductive matrix in order to sufficiently reduce bulk resistance and achieve reasonable electro-discharge machining cutting rates. U.S. Pat. No. 4,507,224 teaches that additions above 20 weight percent cause an apparent rapid decrease in strength. Thus, electroconductive additions are limited to 20 weight percent or less and, in turn, the rate of electro-discharge machining is likewise limited.
In electro-discharge machining the electrically conductive workpiece or ceramic blank is eroded by electric discharges or sparks which on a small scale generate localized shock waves and intense heat. The shock waves and intense heat thermally erode the adjacent workpiece surface. The thermal erosion process comprises the separation of solid particles through melting and vaporization selectively of compounds in the workpiece or ceramic blank. The violence of the process generates micro-cracks in the workpiece surface as particles are removed and cavitation occurs in the dielectric liquid surrounding the workpiece and electro-discharge machining electrode.
The generation of surface micro-cracks is detrimental to strength in the ceramic if the surface cracks exceed about 50 microns in depth. Appreciable useful strength is lost during the electro-discharge machining of electro-conductive ceramics as reported by Firestone, SME Technical Paper MR 87-112,1987. A limitation on thin-wall sections of about 0.040 inches due to the fragile nature of the post electro-discharge machined sections has been reported by Dauw, The Machining of Electrically Conductive Ceramics by EDM, Worldwide Engineering Services Meeting 1988, Ferney-Voltaire (France).
Micro-crack initiation and propagation in ceramic materials may be significantly reduced as disclosed in U.S. Pat. No. 4,543,345 (Re 32,843). This patent also identifies means of achieving properly dispersed SiC whiskers in ceramic powders as does U.S. Pat. No. 4,463,058. Known prior art patents, however, do not discuss nor disclose the microscopic aspects of fracture toughness in ceramic composites in relation to electro-discharge machining nor do they discuss toughness levels obtained in the ceramic composites disclosed.
The ceramic materials disclosed above are very hard subsequent to sintering or hot pressing therefore final shaping is generally limited to diamond grinding and ultrasonic machining. Diamond grinding and ultrasonic machining are costly and limited to relatively uncomplicated shapes. Furthermore the addition of electrically conductive ceramic materials typically compromises the fracture toughness and strength of the ceramic. For purposes of ceramic heaters and glow plugs some compromise of toughness and strength is permissible. Resistance to high temperature and chemical degradation are usually of more importance. For cutting inserts, wear parts and die tooling, however, fracture toughness and strength become paramount along with resistance to chemical attack. Where wear resistance, resistance to high temperature degradation and chemical attack, strength and fracture toughness are all required along with the ability to form accurate intricate shapes in the ceramic, the problems combine. Such combinations are required for high temperature extrusion and wire drawing dies in particular as well as cutting tools and wear parts for high temperature and corrosive applications.
It would therefore be highly advantageous to produce a ceramic composite material capable of being electro-discharge machined to final shape at high cutting rates for economic reasons. The new composite material should be sufficiently thermal shock resistant to minimize surface damage generated during the electro-discharge machining process while retaining or improving high bulk mechanical properties, in particular high fracture toughness.